In recent years, accompanying with the popularization of a lithium ion secondary battery, there is an increasing demand for a battery which is capable of operating at higher voltage and has a higher energy density.
In general, a graphite-based material is used for a negative electrode of a lithium ion battery. Because a lithium ion is inserted into graphite layers so as to form the composition of LiC6, the theoretical capacity of a general graphite-based material is 372 mAh/g (LiC6). For this reason, the energy density has substantially reached the limit in a conventional lithium ion battery. In order to further improve the energy density of a lithium ion battery, it is necessary to select a new material. Therefore, the attention is payed to the materials that are alloyed with lithium such as silicon and tin. These materials have a lower potential than carbon and lithium, and have a large specific capacity.
Among these materials, silicon can insert lithium up to 4.4 lithium atoms with respect to 1 silicon atom in a molar ratio. Therefore, silicon can theoretically have 10 times the capacity of a graphite material. However, when a silicon particle inserts lithium atoms, the volume swells by about 3-4 times. For this reason, there are the problems that the repetition of charge and discharge cracks and pulverizes silicon particles, and affects the other members constituting the electrodes. In terms of suppressing the pulverization of silicon particles, it is effective to reduce a particle size. However, when reducing a particle size, aggregation is likely to occur. Therefore, the measures such as the coating a silicon particle with a silicon oxide or a carbonaceous material is implemented. However, the decrease in capacity due to repeated use is not sufficiently suppressed even in the aforementioned coated silicon particle.
On the other hand, it is known that a charge and discharge behavior of a lithium atom is shown even in the electrode obtained by using a silicon dioxide as an active material.
A detailed mechanism of charge and discharge by the silicon oxide is not yet known. Because the volume change of a silicon oxide is smaller than that of elemental silicon, the improvement of cycle life is expected. However, in the electrode in which an electrode active material is made of silicon oxide alone, the theoretical capacity and efficiency are low, and the energy density is small as compared with the electrode in which an electrode active material is made of silicon (Si) and silicon monoxide (SAD).
Also, there is the known active material containing silicon and the silicon oxide represented by the general formula SiOx (1.0≤x<1.6). The active material obtained by using a silicon oxide as a starting material forms the structure, in which silicon particles having a size of several nanometers are enclosed in silicon oxides through a disproportionation reaction caused by a thermal treatment. The fine silicon particles are likely to cause aggregation and grain growth in a charge and discharge process, and the improvement of life has not been achieved yet. On the contrary, cycle life is much shortened by introducing silicon.
Also, there is the known method of coating silicon oxide particles with a carbonaceous material followed by firing. In this method, about 60 mass % of a silicon oxide phase is reduced during the thermal treatment, to thereby produce the composite particle of the silicon oxide and the carbonaceous material SiOx (0<x≤1.5). According to this method, the cycle characteristics are well improved. However, regarding the silicon oxide particle having the composition of SiOx, it is consequently difficult to extend the cycle life for the same reasons as the aforementioned case of a composite material of silicon oxide and silicon.